Resonant ultrasound spectroscopy and acoustic interferometry are two techniques that use temporal-frequency information, such as resonant frequencies, as fingerprints for identifying and/or characterizing inspected specimens. Such information is derived from full-body responses of the specimen, so they can provide bulk properties such as elastic moduli or density. They can also use these fingerprints to identify specimens that are dissimilar. However, these techniques are unable to provide local information about specimens, such as where an irregularity is, what type it is, or how large it is.
The most common forms of ultrasonic inspection utilize short-duration ultrasonic pulses for interrogation. A short duration pulse will propagate through the specimen being inspected, speeding up, slowing down, attenuating, reflecting, and scattering, depending on the composite of the specimen. Measurements of the wave after it has passed through the specimen allow one to discern the properties/structure of the specimen.
When making measurements at many inspection points, an inspection system must wait for each previous excitation pulse to sufficiently disperse in the specimen or in the surrounding fluid (air or otherwise) so as to prevent it from significantly affecting subsequent measurements. Excitation and sensing may be provided by using roving vibration transducers on or in proximity to the specimen or through optical means such as Q-switched lasers for excitation and laser interferometry or Doppler vibrometry for sensing.
State of the art remote inspection systems utilize a laser for excitation and a laser for sensing. A pulsed laser provides the short-duration excitation through local thermo-elastic expansion on the specimen. The pulse energy is limited to the laser damage threshold of the specimen being inspected. A laser Doppler vibrometer (LDV) then measures the response at each inspection point. Either or both lasers may be scanned over the structure. The low sensitivity of the LDVs in the ultrasonic range often means that multiple measurements must be made at each inspection point and averaged, further reducing the scan rate.
Continuous scanning laser Doppler vibrometery (CSLDV) has been used to rapidly measure the operating deflection shape (ODS) of inspected systems using continuous excitation. The scanning, acquisition, and processing techniques, however, limit systems to relatively low frequencies, which means only global system vibration responses can be measured, providing no local information on specimen properties/structure. Therefore, ample opportunity exists for improvements to measurement technologies.